Noble gas mass spectrometry is important for radiometric dating or isotope geochemistry, for example argon-argon dating and helium or xenon isotope analysis. Noble gas mass spectrometry usually uses a static gas mass spectrometer in which a gaseous sample containing the noble gas or gases of interest is fed into a mass spectrometer and then left in the spectrometer without pumping during the mass analysis. A characteristic feature of static mass spectrometers is therefore that they stay evacuated during analysis. Static mass spectrometers are used when a very high degree of sensitivity is required. Analysis is typically conducted for detecting the presence of minute quantities of noble gases (He, Ne, Ar, Kr, Xe), although static mass spectrometers may also be capable of analyzing other gases, such as CO2 or N2 for example. Examples of such instruments include the Helix™ and Argus™ instruments from Thermo Scientific™.
In more detail, in noble gas isotope ratio mass spectrometry, the sample gas is prepared, typically from a solid sample, in a sample preparation gas line, which for instance can be connected to a means of heating, such as a furnace at high temperature or a laser heating device, to release small amounts of sample gas trapped inside a solid sample, such as a small crystal or mineral(s) by sample heating. The sample gas comprises one or more noble gases to be analyzed. In other cases the noble gas can also be obtained and/or cleaned up from a gas sample directly, for instance from an air gas sample.
One or more traps, such as cold traps, and/or chemical getters, such as getter pumps, are used in the preparation line for sample gas clean up. The traps and/or getters act to remove active gases from the sample gas, thereby leaving the inert noble gases for analysis. During sample preparation, typically very small amounts of noble gases (for example ca. 1 μl at standard pressure, or less) are released from the solid sample and cleaned up with the chemical getters and cold traps before the gas is introduced into the evacuated mass spectrometer by opening an inlet valve to the mass spectrometer. The gas may be admitted from a vacuum of around 10−4 mbar in the sample preparation line to a high vacuum or ultra high vacuum in the mass spectrometer.
The time at which the gas is introduced into the mass spectrometer is defined as “time zero” in the prior art. Before introduction of the gas into the evacuated mass spectrometer, the vacuum pumps of the spectrometer are isolated from the spectrometer such that no gas is pumped from the mass spectrometer during the analysis time. Thus, the analysis is performed under a static vacuum condition. The static vacuum condition requires a sealed mass spectrometer (preferably sealed to high or ultra high vacuum) having clean internal surfaces with very low outgassing rates (usually as a result of a bake-out procedure).
The ionization of the sample gas in the static gas mass spectrometer is usually achieved using electron impact ionization inside an ionization volume of a Nier type ion source. The ionized noble gas species are extracted out of the ionization volume by electric fields and accelerated into the mass analyser, which usually is a magnetic sector mass analyser, but it could alternatively be another type, such as a quadrupole mass analyser or a time of flight mass analyser. A multicollector, for example comprising a plurality of Faraday cups and/or electron multipliers (usually a combination of the two types), is usually used for detection of the ions, in particular with the preferred magnetic sector mass analyser.
In the analysis, isotope abundances and typically one or more isotope abundance ratios are measured and the measured data is subsequently extrapolated back to “time zero”, the time when the sample gas was first introduced into the mass analyser in order to account for consumption and isotope fractionation by ionisation of the gas during measurement.
It is important to capture all measured isotope ratios starting from “time zero” in order to deduce the accurate isotope ratio from the measured data. However, there are problems with the known measurement methods and apparatus. The ionizing electron beam inside the ionization volume of the electron impact ion source is generated from a hot filament by thermionic electron emission. The ion source conditions have to be kept stable over time in order to avoid any distortion of the measured isotope ratios. For example, changes in filament temperature during sample measurement would result in uncontrolled isotope fractionation and affect the accuracy and precision of the measurement. Changes in filament current during the measurement might influence the space charge conditions inside the ionization volume and thus affect the mass discrimination of the ion source. Furthermore, there is an initial equilibration time or period, starting from time zero until the different isotopes have evenly spatially dispersed throughout the volume of the mass spectrometer. Because of the increased viscosity, this equilibration time can last longest for the heavier noble gases such as Xenon, which can take several minutes (e.g. up to 10 minutes) before all isotopic species of the noble gas sample have been fully equilibrated from the sample prep line into the volume of the mass spectrometer. For example, equilibration can take about 3 minutes for argon, or 6 to 7 minutes for Xenon. The equilibration time will depend on characteristics of the particular instrument as well as of the gas.
As a result of the equilibration time, the measured isotope abundance over time can show a behaviour from time zero, t0, as shown schematically by curve 4 (solid line) in FIG. 1. There is typically a fast rise 6 in the measured isotope abundance as the isotopes fill the spectrometer following their introduction, which is followed by a decrease, for example a linear or substantially linear decrease 8 after a time teq as the isotopes are gradually consumed. The ionization source itself causes fractionation of the isotopes and therefore a change in isotope ratio with time. Space charge effects and the different kinetics of the lighter compared to the heavier isotopes result in slightly different transmission and ionization probabilities. Because of preferential ionization of one isotope over another, the isotopic composition of the gas inside the evacuated system changes over time and, as such, the measured isotope ratio changes over time. In order to calculate the true isotopic composition of the sample it is important to measure all isotopes right from the point of introduction of the sample. Therefore, an extrapolation 7 of the stable, equilibrated, and decreasing part of the isotope intensity measurement back to time zero (moment of gas introduction) is required to calculate an isotope ratio of the gas at time zero. Curve fitting is performed to extrapolate the isotope intensity to time zero. In the state of the art, typically measurement of the isotopes will only begin after the equilibration phase, i.e. in the stable, linear behaviour phase, even though the gas has been subject to ionization since it was first introduced into the spectrometer. For example, as shown schematically in FIG. 2, measurements for an argon sample typically may not be taken before 200 seconds have passed since the gas was introduced and a linear behaviour is observed. Usually the intensity, i.e. abundance, of the isotopes is measured over time, rather than the ratio. A best fitting of the measured ion beam intensity is performed and extrapolated back to time zero for each isotope. An isotope ratio is then calculated at time zero from the ratio of the time zero intensities of two isotopes.
As mentioned, the period before the observed linear decrease in isotope abundance commences corresponds to the equilibration time and is most evident for the heavier noble gases, e.g. argon to xenon. In state of the art systems, in order to keep ion source conditions stable, ionization of the sample gas begins from the moment the sample is introduced into the mass spectrometer vacuum. However, this means that the isotopes are being consumed in an uncontrolled and unknown way so that the isotope abundance ratios are disturbed by the time the gas is equilibrated (the initial isotope ratio measured in the equilibration time would not be consistent with the measured isotope ratios in the equilibrated period). Furthermore, the ionisation and consumption of gas during the equilibration phase is another source of isotope fractionation. This fundamental limitation today causes one of the major uncertainties in noble gas isotope ratio mass spectrometry.
The invention is aimed at addressing this problem, amongst others.